PROBLEM TO BE SOLVED: To detect failures related to a steering pump and prevent vehicle turning unless actual turning is commanded by a driver. SOLUTION: When a steering control signal indicates that a steering wheel 74 is turned in a direction opposite to the direction of rotation of a steering motor 42, a control unit 70 saves a first ratio value representing a ratio of the motor speed to the pump speed. When the signal is unchanged after a certain time duration, the control unit 70 saves a second ratio value representing a later ratio of the motor speed to the pump speed. The control unit then calculates a difference between the first and second ratio values, and generates a fault signal if the difference has a magnitude which is less than a predetermined magnitude. Thus, the control system generates a fault signal when the sensed signal indicates that the position of a steering pump swashplate is not changing as it should in response to changes in steering wheel position.